This invention relates to a braking pressure control unit for a dual-circuit vehicle brake system actuatable by pressure fluid, wherein the relative movement between a closure member furnished with a closure spring and a valve seat of a valve assembly arranged in a housing between inlet and outlet of a first brake circuit is controlled by a stepped piston that is loaded by a control force. The valve seat is fixed with the housing. The stepped piston is located on the side of the valve seat remote from the closure member, and the closure member is actuatable by virtue of an extension of the stepped piston penetrating the valve seat. The relative movement is controllable by a malfunction piston which is slidable in a housing bore in a sealed relationship thereto and whose first effective surface is acted upon by the pressure in the first brake circuit and whose second effective surface is acted upon by the pressure in the second brake circuit. The closure spring is arranged between closure member and malfunction piston and the closure member is movable to bear against a stop of the malfunction piston.
A braking pressure control unit of this type is known from German printed patent application No. 30 17 256. Accommodated in the housing of this braking pressure control unit is the valve seat, with which a closure member cooperates. This closure member is loaded by a closure spring which is supported on the malfunction piston. The closure member is movable by the force of the closure spring to bear against a stop on a peg-shaped extension of the malfunction piston and embraces it with resilient arms. The valve can be prevented from closing by the tappet at the stepped piston penetrating the valve seat and by the malfunction piston coupled to the closure member. The malfunction piston is on both of its end surfaces acted upon by the pressure of a brake circuit and displaces towards the valve seat in the event of pressure build-up. A prop ring is movable into abutment on a stop at the closure member, on which prop ring one end of a restoring spring is supported which, with its other end, is supported on the malfunction piston and which is anchored between these components. The prop ring abuts on a housing area adjacent to the valve seat. When the brake system is unpressurized, the closure member is spaced from the tappet at the stepped piston. Upon relative movement of the closure member towards the valve seat, the closure member is lifted from the stop on the prop ring and is moved to bear against the valve seat or against the tapped of the stepped piston. In the event of pressure decrease, the closure member is moved by the restoring spring to abut on the prop ring. This braking pressure control unit entails substantial design efforts because of the provision of two anchored spring arrangements with their associated stop devices.
It is an object of the present invention to arrange a braking pressure control unit of the type described with a minimum number of components, while its principal mode of function is preserved.